1. Field
The present invention relates generally to radar systems, and, in particular to radar systems comprising direct-to-digital receive signal conversion.
2. Description of the Problem and Related Art
RF communications systems, including radar systems, employ a superheterodyne-generated wave modulated with an information signal that must be down-converted by a receiver to extract the information signal. It has been necessary to convert the RF energy that is received at the receiver antenna into a format that can be interpreted by a human, or computer, into a meaningful target image or display. This has been accomplished by down-converting the RF energy through one or more IF stages into a baseband frequency, at which point the target could be displayed on an oscilloscope or similar device. Conventional systems use an ever-increasingly complex series of amplifiers and filters to convert a received signal from an intermediate frequency (IF) signal to a baseband signal. Most such systems employ a local oscillator (LO) to essentially provide a reference signal of matching frequency to that received at the antenna in order to provide a template by which the received signal is multiplied, or correlated, with a mixer and then filtered.
Following the widespread availability of digital technology, the baseband signal was converted to digital formats by one or more ADCs and sent to a radar signal processor (RSP). The conversion of the baseband signal to digital format allowed for the rapid growth in complexity of modern radar. Some recent receiving process techniques have sought to bypass the IF stage through a direct conversion technique; however, even this class of receivers requires the use of an LO. The use of an LO still introduces a number of sources of error, among which include spurs, image-rejection issues, additional complex filtering, etc.
In recent years the separation between radar and communications hardware and software has become blurred to the point that modern radars use many of the same modulation techniques as modern radios. Of particular interest is the class of Orthogonal Frequency-Division Multiplex (OFDM) waveforms used in the cellular phone industry applied now to radar, and specifically to Ground Penetrating Radar.
As the complexity of modern radars and their waveforms increase, so also does the complexity of the receiver hardware, and the duration of the engineering design cycle. The concept of software-defined radio (“SDRadio”), gained prominence, the goal of which is to ultimately eliminate all receiver hardware and couple the receive antenna directly to a fast analog-to-digital converter (ADC). However, that goal has yet to be achieved in the domain of SDRadio. In the paper “Software-defined radio receiver: dream to reality”, Bagheri, et. al., [29] show the continued reliance on the LO and mixers within the SDRadio architecture. The most recent research on SDRadio reveals the persistent use the LO and mixer hardware.
Likewise, elimination of hardware components and the adaptability of the system, has become a goal of radar design. But, such prior solutions still require RF down-conversion with the corresponding LO and mixer hardware.
This invention eliminates the need for radar receiver hardware, specifically the Local Oscillator (LO) and radio frequency (RF) and Intermediate Frequency (IF) Mixers, that have classically been employed since the inception of radar.